1. Field of the Invention
The present invention relates to a multi-layer refractory structure able to withstand high temperatures of the order of 2000.degree. C. to 2500.degree. C. for hundreds of hours. Although not exclusively, it is particularly appropriate for the formation of refractory linings for ducts intended to convey hot gas flows and it will be more especially described hereafter in this application.
2. Description of the Prior Art
It is known that such ducts are used in numerous industrial installations employing gases at very high temperatures, such more particularly as furnaces, steel and iron plants or coal distillation plants.
For conveying high temperature gas flows, the prior technique is familiar with two types of ducts:
on the one hand, metal ducts cooled by a fluid, such as water; these ducts may withstand high temperatures but, because of the liquid cooling, they are complicated to manufacture, entail considerable constaints and are the source of considerable heat losses. Thus, such ducts are not only costly to manufacture and maintain but they are also the cause of low heat yields;
on the other hand, metal ducts, not cooled by a fluid, comprising an internal refractory lining; these ducts overcome the drawbacks of the cooled ducts, but on the other hand cannot be used for conveying very high hot gas flows, because of the poor heat resistance of known refractory linings. In practice, such uncooled ducts cannot be used when the temperature of the gas flows exceeds 1300.degree. C.
Now, new heating means are known at the present time, such as plasma generators, providing very high temperatures and it is often advantageous in certain industrial processes to increase the operating temperatures so as to obtain more complete and/or more rapid reactions.
For example, it is often desirable to increase as much as possible the temperature of the blowing gas in a blast furnace, so as to increase the production and reduce the amount of coke required for operating said blast furnace.
To this end, it is already known to equip a blast furnace with a plasma generator and to connect the output of said plasma generator to the duct feeding the blowing gas to the nozzle injecting it into said blast furnace. Such a technique is described for example in the patents FR-A No. 2 223 449, FR-A No. 2 223 647, GB-A No. 1 488 976, U.S. Pat. No. 4,363,656 and FR-A No. 2 515 326.
However, this technique raises practical difficulties because of the very high temperature (several thousand .degree.C.) of the plasma generated by the generator. Penetrating into the duct of the blowing gas and coming into contact with the walls thereof, the plasma causes accelerated wear and destruction of said walls. In U.S. Pat. No. 4,363,656, this drawback has already been noted in connection with the technique described in the patent GB-A No. 1 488 976 and to overcome it, it has already been proposed to slant the axes of the plasma generator and of the blowing gas duct with respect to the axis of the nozzle injecting into the blast furnace. The result is a bend between the blowing duct and the nozzle which may generate disturbances in the flow of the blowing gas. In addition, such a solution is not easy to implement for improving a preexisting blast furnace.